Influence of Supplemental, Dietary Vitamin A on Retinol-Binding Protein Concentrations in the Plasma of Preruminant Calves

¹ Periparturient Diseases of Cattle Research Unit, USDA, ARS, National Animal Disease Center, Ames, IA, 50010-0070
² Department of Animal Science, University of Tennessee, Knoxville, 37901-1071
³ Dairy Science Department, South Dakota State University, Brookings, SD 57007

Transport of retinol (vitamin A alcohol) from retinoid stores in the liver to target tissues is accomplished exclusively by a specific plasma protein, retinol-binding protein. Within individuals, retinol-binding protein concentrations in plasma are regulated and remain constant except in extremes of vitamin A nutriture or in disease. In the present study, retinol-binding protein concentrations in plasma from preruminant calves supplemented with 0, 1700 (i.e., current NRC requirement), 34,000, or 68,000 IU of vitamin A daily from birth to 27 d of age (n = 6/treatment) were quantified. Retinol-binding protein concentrations at birth averaged 21 µg/ml (n = 24) or approximately 50% of concentrations in dairy heifers and cows. Plasma retinol and retinol-binding protein concentrations were correlated positively, corroborating the role of vitamin A nutriture in the regulation of retinol-binding protein secretion from the liver. In this regard, dietary vitamin A influenced positively retinol and retinol-binding protein concentrations and, as a consequence, the degree of saturation of retinol-binding protein with retinol. At 27 d of age, calves fed 34,000 IU of vitamin A had substantially higher retinol and retinol-binding protein concentrations than did calves fed 1700 IU of vitamin A, indicating that dietary vitamin A effects positively vitamin A status. The data also suggest that the current NRC requirement may not be sufficient to assure vitamin A adequacy in preruminant calves. Percent saturation of retinol-binding protein with retinol in all calves was <35%, much lower than anticipated and suggests that the retinol requirement of vitamin A-responsive tissues exceeded vitamin A availability.

Key Words: preruminant calf • vitamin A • retinol-binding protein

Submitted on July 10, 2000
Accepted on October 23, 2000